More natural syntax for histogram constructor

One can now write :

  Histogram<2, atomicity::full, float> hist{&algo, "Test2DHist", "Test 2D histogram", {64, 0., 64.}, {52, 0., 52.}};

while it used to be only

  Histogram<2, atomicity::full, float> hist{&algo, "Test2DHist", "Test 2D histogram", {{64, 0., 64.}, {52, 0., 52.}}};

with extra braces.